By-products of metallurgical processes, such as slags which result from the smelting of both ferrous and non-ferrous ores, and combustion products of coal from fossil fuel powered generating stations, such as fly ash or clinker, all represent the product of a significant investment of energy which is normally lost upon the subsequent disposal of these materials, heretofore considered as waste.
The term `slag` as used in this application refers to the vitreous mass which remains after the smelting of a metallic ore, a process which entails the reduction of the metallic constituents in the ore to a molten state.
The terms `ferrous slag` or `blast furnace slag` refer to that slag which remains after the smelting of iron ore. Alternately, `non-ferrous slag` is that slag which remains after the smelting of a non-ferrous ore such as copper, nickel, lead, or zinc, whether it be done in a blast furnace or otherwise. Therefore, a slag considered non-ferrous in the metallurgical sense may contain appreciable amounts of iron due to the presence of this compound as an impurity within the ore.
Fly ash is a combustion product resulting from the burning of coal which has been ground to a relatively fine particle size. Coal which is not as finely ground produces both fly ash and a coarse, incombustible residue known as clinker.
Iron blast furnace slag, fly ash and various `natural` slags and ashes have been known for some time to have pozzolanic properties. Waste industrial slags, however, do not contain the correct quantities of essential ingredients of Portland cements. Iron blast furnace slag comes closest with a CaO/SiO.sub.2 ratio of approximately 1. To acquire such pozzolanic properties, however, such slags must be cooled to an amorphones (or highly vitreous) state by rapid quenching, such as by immersion in a large quantity of high pressure water. It is well known for example, that granulated blast-furnaces lag obtained in the production of pig iron can be mixed with Portland cement clinker and the mixture finely ground to bring out the inherent hydraulic properties of the slag.
In most applications today where ferrous slag or fly ash is used in backfilling, one finds a combination of a small quantity of Portland cement providing some initial set and strength, and slag or fly ash, which reacts with residual CaO from the cement reactions to form calcium silicates which provide the balance of the strength required over a period of time. To enable the use of non-ferrous slags as cement, however, the chemical composition of typical nonferrous slags must generally be modified to produce a product capable of competing with Portland cement, i.e. to one capable of achieving high early strength as well as a high ultimate strength within an acceptable time frame.
Applicants have determined that a higher quality, less expensive cemented backfill may be produced, without the use of Portland cement, by adjusting the proportions of both CaO and Al.sub.2 O.sub.3 in non-ferrous slag. This adjustment then converts the slag from a `low-grade` pozzolan to a `high-grade` pozzolan wherein the necessary aluminate reaction can be harnessed by adjusting the sulphate ion and calcium hydroxide concentrations. Further, the methods for making cement from non-ferrous slag may also be extrapolated and used to prepare improved cements utilizing ferrous slags or fossil fuel combustion residues.